If the SIMs are unlocked you can put them in another phone and indeed use that one instead. However, they can easily find that out because they see exactly what type of phone you use based on IMEI. There is nothing you can do about that. What their response will be, I do not know.

About tethering in the US with your provider ATT I don't know. It should work with any provider, but you will need to activate BlueTooth DUN on N900 which is not officially supported by Nokia.